Major efforts have been devoted over the last ten years to the development of light emitters able to emit photons one by one in a deterministic way. Such emitters, known as single photon sources (SPS), are key devices in quantum cryptography, and could find applications in quantum information processing or metrology (standards for the energy or the light flux).
The spontaneous far-field radiation pattern of a single photon emitter is naturally omni-directional. In view of practical applications, it is therefore essential to tailor this radiation pattern so as to ensure, e.g., a directional emission in free space, or a good coupling to an optical channel, such as an optical fibre. Generally speaking, such a tailoring is performed through an engineering of the electromagnetic environment of the emitter.
For quantum dots (QDs), the standard approach consists in embedding the quantum dot in an optical microcavity supporting discrete resonant modes, these modes having a low mode volume and a high quality factor. If the frequency of the quantum dot emission corresponds to one of the resonant modes frequencies (quantum dot and cavity mode in resonance), the spontaneous emission rate of the quantum dot into the cavity mode is strongly enhanced: this is the so-called Purcell effect. Due to this preferential coupling, a large fraction beta (beta˜1) of the quantum dot spontaneous emission is funnelled into this resonant cavity mode. The emission diagram of the quantum dot-cavity system is then defined by the geometry of the microcavity. Assuming that a fraction (eta) of this radiation pattern is collected by optics elements, the source efficiency (epsilon), a key figure of merit for all potential applications, is the product beta*eta.
The Purcell approach has been successfully demonstrated with micropillars or two-dimensional photonic crystal cavities.
In the reference US2003/0152228 an optoelectronic component is disclosed which is capable of emitting light pulses containing a single photon comprising an optical resonant cavity and a photon emitting unit placed in said optical cavity.
However, the far field emission pattern of the required high-Q cavity is very sensitive to fabrication imperfections. Optimizing epsilon thus implies a trade-off between beta and the collection efficiency eta. Despite the significant technological progresses of the last years, epsilon remains limited to values around 40%. This limitation is even more serious in electrically pumped devices, because of supplementary optical losses introduced by the cavity doping. So far, the best reported efficiency in an electrically driven single photon source based on a cavity design is of about 14%, well below unity. Besides this drawback, the Purcell approach is only effective over the narrow bandwidth of the cavity resonance and is limited to monochromatic emitters, such as single quantum dots at cryogenic temperatures (T<100 K).
The reference “Single quantum dot nanowire LEDs”, Minot E D et al., Nano Letters February 2007 American Chemical Society US, vol. 7, no. 2, pages 367-371 disclose fabrication of InP-InAsP nanowire light-emitting diodes in which electron-hole combination is restricted to a quantum-dot-sized InAsP section. The reference shows electroluminescence properties of an InP nanowire light emitting diode, and not a single photon source.
The reference “Optics with single nanowires”, Zwiller V et al., Comptes Rendus—Physique, Elsevier, Paris, FR, vol. 9, no. 8, 1 Oct. 2008, pages 804-815, ISSN: 1631-0705 describes that heterostructures in nanowires can define quantum emitters and that single spins can be addressed optically. It also presents results on electrically contacted nanowires. The reference discloses a number of different embodiments from different academic research groups. The reference does not describe an effective, electrically driven single photon source.
The inventors of the present invention have appreciated that an improved single photon source is of benefit, and have in consequence devised the present invention.